Device and method for producing a dynamic reference signal for a driver circuit for a semiconductor power switch

ABSTRACT

A device ( 442 ) for producing a dynamic reference signal (U REF ) for a control circuit for a power semiconductor switch comprises a reference signal generator ( 442 ) for providing a dynamic reference signal (U REF ), which has a stationary signal level after elapse of a predefined time following a switching process of the power semiconductor switch, a passive charging circuit ( 450 ) which is configured to increase a signal level of the dynamic reference signal in reaction to a switching of a control signal of the power semiconductor switch from an OFF state to ON state for at least one part of the predefined time above the stationary signal level, in order to produce the dynamic reference signal and an output (A) for tapping the dynamic reference signal (U REF ).

TECHNICAL FIELD

The present invention relates to a device and a method for producing avariable reference signal for a driver circuit for a semiconductor powerswitch. Such devices are used for example for detecting a short-circuitor overcurrent state in an IGBT (“Insulated Gate Bipolar Transistor”).

BACKGROUND

In some power semiconductor switches, high voltages can be applied andhigh currents can be carried. Short circuits or excessively increasedcurrents can therefore rapidly lead to the thermal destruction of thepower semiconductor switches. Therefore, power semiconductor switchescan have protective circuits for detecting a short-circuit orovercurrent state. One possibility for detecting these states is theindirect monitoring of the current through the power semiconductorswitch on the basis of a voltage dropped across the power semiconductorswitch. After the power semiconductor switch has been switched on, saidvoltage should fall quickly from a relatively high level in theswitched-off state (the “switched-off state” or “OFF state” is a stateof the power semiconductor switch in which the latter is “open” andcarries no current) to a relatively low level in the switched-on state(the “switched-on state” or “ON state” is a state of the powersemiconductor switch in which the latter is “closed” and can carrycurrent). Correspondingly, a control signal of a power semiconductorswitch (for example a gate-emitter driver signal) has an ON state, inwhich it keeps the power semiconductor switch closed, and an OFF state,in which it keeps the power semiconductor switch open.

The solid curve 678 at the bottom left in FIG. 6 shows an exemplaryprofile of a collector-emitter voltage of an IGBT (an IGBT is anexemplary power semiconductor switch) during the switchover process froma switched-off state into a switched-on state (the profile of anassociated exemplary control signal 630 is illustrated at the top left).As illustrated, the collector-emitter voltage falls sharply to a verylow value (close to 0 volts). An exemplary short-circuit behavior of anIGBT is illustrated at the bottom right in FIG. 6 (solid curve 678). Incontrast to normal operation, the collector-emitter voltage does notfall to the very low value; simultaneously, however, high currents canflow in the IGBT (for example between three times and ten times thenominal current of the IGBT). In other short-circuit cases, thecollector-emitter voltage indeed firstly falls to the value in normaloperation, but then rises again. This results in a high thermal loadingof the power semiconductor switch, which can incur damage after arelatively short time. In this regard, for example, some IGBTs in theswitched-on state withstand a short circuit for approximately 10 μswithout incurring damage. Therefore, the protective circuits fordetecting a short-circuit or overcurrent state in this time range canensure that the power semiconductor switch is switched off. Similarcharacteristics can also be found in other power semiconductor switchesbesides IGBTs. An overcurrent state, like a short-circuit state, can bemanifested by an increased collector-emitter voltage. However, in theovercurrent case, the collector-emitter voltage can be closer to acollector-emitter voltage in the normal case in comparison with theshort-circuit case.

The outlined differences in the profile of the collector-emitter voltagebetween normal operation and the short-circuit and/or overcurrent casecan be utilized in protective circuits for detecting a short-circuit orovercurrent state, in order to detect a short-circuit or overcurrentstate. It is thus possible to define a threshold value for thecollector-emitter voltage, which threshold value is used to detect thepresence of a short-circuit or overcurrent state. By way of example, ifthe collector-emitter voltage rises above the threshold value (see FIG.6, bottom right), a short-circuit or overcurrent state can be detected.

SUMMARY OF THE INVENTION

A first device for producing a dynamic reference signal for a controlcircuit for a power semiconductor switch in accordance with oneembodiment of the invention comprises a reference signal generator forproviding a dynamic reference signal having a steady-state signal levelafter a predetermined time has elapsed after a switchover process of thepower semiconductor switch, a passive charging circuit, which isconfigured to increase a signal level of the dynamic reference signal inreaction to a switchover of a control signal of the power semiconductorswitch from an OFF state into an ON state for at least one part of thepredetermined time above the steady-state signal level, in order toproduce the dynamic reference signal, and an output for tapping off thedynamic reference signal.

Such a device makes it possible to detect a short-circuit and/orovercurrent state in a power semiconductor switch reliably and alsorapidly enough. In particular, the probability of the erroneousdetection of a short-circuit and/or overcurrent state can be reduced incomparison with devices having a steady-state reference signal. This isachieved by the momentary increase in the level of the reference signalfor at least one part of a predetermined time in reaction to aswitchover of a control signal of the power semiconductor switch from anOFF state into an ON state. At the same time the device of the inventioncan bring about the increase in the level of the dynamic referencesignal without active components. The device can thus be morecost-effective and more robust than comparable devices having activecomponents. In some examples, the device can be configured reliably andwith a negligibly small temperature drift, with comparatively littleoutlay.

An exemplary profile of a dynamic reference signal is shown at thebottom left in FIG. 6 (dashed line 648). The level of the dynamicreference signal is increased abruptly in reaction to a switchover of acontrol signal of the power semiconductor switch from an OFF state intoan ON state and thereupon falls continuously until the steady-statelevel is reached again. This increased reference voltage following aswitchover process from an OFF state into an ON state can be used in aprotective circuit. The protective circuit does not trigger since asignal (U_(C′E)) representing the collector-emitter voltage alwaysremains below the reference signal.

In a second device in accordance with the first device, the dynamicreference signal is a dynamic reference voltage.

In a third device in accordance with the first or second device, thepower semiconductor switch is an IGBT.

In a fourth device in accordance with any of the preceding devices, thepassive charging circuit comprises an RC element.

In a fifth device in accordance with the fourth device, the passivecharging circuit is configured such that the control signal of the powersemiconductor switch can be coupled to a first terminal of a capacitanceof the RC element and a second terminal of the capacitance of the RCelement is coupled directly or indirectly to the output for tapping offthe dynamic reference signal.

In a sixth device in accordance with any of the preceding devices, thepassive charging circuit is configured to increase the signal level ofthe dynamic reference signal in reaction to the switchover of thecontrol signal in the manner of an abrupt jump.

In a seventh device in accordance with the fifth or sixth device, thepassive charging circuit is further configured such that the capacitanceof the RC element charges following a switchover of a control signal ofthe power semiconductor switch from an ON state into an OFF state.

In an eighth device in accordance with the fourth device or the fourthdevice and any of the fifth to seventh devices, the passive chargingcircuit is configured such that the capacitance of the RC elementdischarges following the switchover of the control signal and thusproduces the temporarily increased level of the dynamic reference signaland returns it to the steady-state level again as a result of thedischarge.

In a ninth device in accordance with the eighth device, a 1/e timeconstant of the RC element is greater than 1 μs.

In a tenth device in accordance with the eighth device, a 1/e timeconstant of the RC element is between 5 and 15 μs.

In an eleventh device in accordance with any of the preceding devices,the reference signal generator contains a circuit for producing thesteady-state signal level.

In a twelfth device in accordance with the eleventh device, the circuitfor producing the steady-state signal level comprises one or moreresistors and an input for receiving a constant current, wherein thecircuit for producing the steady-state signal level is configured toconduct the constant current through the one or more resistors, suchthat a steady-state reference voltage is dropped across the one or moreresistors in order to produce the steady-state signal level.

In a thirteenth device in accordance with the twelfth device, the one ormore resistors are coupled to a first internal reference level.

In a fourteenth device in accordance with the thirteenth device, whereinthe first internal reference level corresponds to an emitter voltage, acathode voltage or a source voltage of the power semiconductor switch.

In a fifteenth device in accordance with any of the preceding devices,the device further comprises a first clamping circuit, which isconfigured to limit the dynamic reference voltage to a predeterminedminimum level.

In a sixteenth device in accordance with the fifteenth device, theminimum level corresponds to an emitter voltage, a cathode voltage or asource voltage of the power semiconductor switch.

In a seventeenth device in accordance with the fifteenth device, theminimum level is less than or equal to the steady-state signal level.

In an eighteenth device in accordance with any of the preceding devices,the device further comprises a second clamping circuit, which isconfigured to restrict the dynamic reference voltage to a predeterminedmaximum level.

In a nineteenth device in accordance with the eighteenth device, themaximum level corresponds to a signal level of the control signal in theON state.

In a twentieth device in accordance with any of the preceding devices,the device further comprises a passive switch, which is coupled betweenthe reference signal generator and the passive charging circuit, whereinthe passive switch is configured to isolate the reference signalgenerator and the passive charging circuit in reaction to a switchoverof the control signal of the power semiconductor switch from an ON stateinto an OFF state.

In a twenty-first device in accordance with any of the precedingdevices, the predetermined time is between 4 and 25 μs.

In a twenty-second device in accordance with any of the precedingdevices, the predetermined time is between 8 and 15 μs.

In a twenty-third device in accordance with any of the precedingdevices, the passive charging circuit is configured to increase a signallevel of the dynamic reference signal in reaction to a switchover of acontrol signal of the power semiconductor switch from an OFF state intoan ON state for the entire predetermined time above the steady-statesignal level in order to produce the dynamic reference signal.

In a twenty-fourth device in accordance with any of the precedingdevices, the dynamic reference signal is a voltage signal.

In a twenty-fifth device in accordance with any of the first totwenty-third devices, the dynamic reference signal is a current signal.

In a twenty-fifth device in accordance with the eleventh device or theeleventh device and any of the preceding devices, the circuit forproducing the steady-state signal level comprises an input for receivinga constant voltage.

In a twenty-sixth device in accordance with any of the precedingdevices, an internal reference level of the device forms thesteady-state signal level.

A first control circuit for a power semiconductor switch in accordancewith one example of the invention comprises a device for producing adynamic reference signal for a control circuit for a power semiconductorswitch in accordance with any of the first to twenty-sixth devices, ashort-circuit and/or overcurrent state detection circuit, which receivesa dynamic reference signal from the device for producing a dynamicreference signal and a signal representative of a collector-emittervoltage, a signal representative of an anode-cathode voltage or a signalrepresentative of a drain-source voltage of the power semiconductorswitch and is configured to compare the dynamic reference signal withthe signal representative of a collector-emitter voltage, ananode-cathode voltage or a drain-source voltage in order to produce afault signal indicating a presence of a short-circuit and/or overcurrentstate in the power semiconductor switch, and a driver circuit, whichreceives the fault signal and is configured to switch off the powersemiconductor switch if the fault signal indicates the presence of ashort-circuit and/or overcurrent state in the power semiconductorswitch.

In a second control circuit in accordance with the first controlcircuit, the short-circuit and/or overcurrent state detection circuitindicates the presence of a short-circuit and/or overcurrent state inthe power semiconductor switch if the signal representative of acollector-emitter voltage, an anode-cathode voltage or a drain-sourcevoltage is greater than the dynamic reference signal.

In a third control circuit in accordance with the first or secondcontrol circuit and the of one of the eleventh to thirteenth devices,the driver circuit further comprises the current source for producingthe constant current.

A first device for providing electrical energy comprises one or aplurality of inputs for connection to a source of electrical energy, oneor a plurality of outputs for connection of one or a plurality of loads,one of the first to third control circuits and a power semiconductorswitch controlled by the control circuit, wherein the device isconfigured to transmit electrical energy by control of the powersemiconductor switch from the one or the plurality of inputs to the oneor the plurality of outputs.

In a second device for providing electrical energy in accordance withthe first device for providing electrical energy, the powersemiconductor switch is an IGBT or a reverse blocking IGBT.

In a third device for providing electrical energy in accordance with thefirst device for providing electrical energy, the power semiconductorswitch is a GTO, an IEGT, a MOSFET or a bipolar transistor.

In a fourth device for providing electrical energy in accordance withany of the first to third devices for providing electrical energy andthe fourth device, the capacitance of the RC element discharges with atime constant selected in order to prevent the short-circuit and/orovercurrent state detection circuit from detecting a short-circuitand/or overcurrent state in normal operation.

In a fifth device for providing electrical energy in accordance with anyof the first to fourth devices for providing electrical energy and thefourth device, the capacitance of the RC element discharges with a timeconstant selected in order to ensure that the short-circuit and/orovercurrent state detection circuit detects a short-circuit and/orovercurrent state in the short-circuit and/or overcurrent case.

In a sixth device for providing electrical energy in accordance with anyof the first to fifth devices for providing electrical energy, thecontrol circuit further comprises a second power semiconductor switch, asecond control circuit for the second power semiconductor switch,wherein the second control circuit comprises a second device forproducing a dynamic reference signal for a control circuit for thesecond power semiconductor switch in accordance with any of the devices1 to 25, a second short-circuit and/or overcurrent state detectioncircuit, which receives a dynamic reference signal from the seconddevice for producing a dynamic reference signal and a signalrepresentative of a collector-emitter voltage, or the signalrepresentative of a collector-emitter voltage, an anode-cathode voltageor a drain-source voltage of the power semiconductor switch and isconfigured to compare the dynamic reference signal with the signalrepresentative of a collector-emitter voltage, an anode-cathode voltageor a drain-source voltage in order to produce a second fault signalindicating a presence of a short-circuit and/or overcurrent state in thesecond power semiconductor switch, and a second driver circuit, whichreceives the second fault signal and is configured to switch off thesecond power semiconductor switch if the fault signal indicates thepresence of a short-circuit and/or overcurrent state in the powersemiconductor switch.

In a seventh device for providing electrical energy in accordance withthe sixth device for providing electrical energy, the first and secondpower semiconductor switches are connected in series and the device isconfigured to the effect that a load is coupled between the first andsecond power semiconductor switches and an input voltage is applied viathe series-connected first and second power semiconductor switches.

In an eighth device for providing electrical energy in accordance withthe seventh device for providing electrical energy, the first and secondpower semiconductor switches are configured to carry a voltage ofbetween 100 V and 15 kV.

A method for producing a dynamic reference signal for a powersemiconductor switch in accordance with one example of the invention,wherein the method comprises producing a steady-state reference signal,receiving a control signal of the power semiconductor switch, increasinga level of the steady-state reference signal in reaction to a switchoverof a control signal of the power semiconductor switch from an OFF stateinto an ON state for at least one part of the predetermined time using apassive circuit, and outputting the dynamic reference signal.

BRIEF DESCRIPTION OF THE FIGURES

Non-limiting and non-exhaustive exemplary embodiments of the inventionare described with reference to the following figures, wherein identicalreference signs refer to identical component parts in the variousfigures, unless specified otherwise.

FIG. 1 shows a device for providing electrical energy to a load.

FIG. 2 illustrates an exemplary control circuit.

FIG. 3 shows an exemplary device for producing a dynamic referencesignal.

FIG. 4A shows an example of a device for producing a dynamic referencesignal.

FIG. 4B shows a further example of a device for producing a dynamicreference signal.

FIG. 5A shows various signals in the device for producing a dynamicreference signal from FIG. 4A.

FIG. 5B shows various signals in the device for producing a dynamicreference signal from FIG. 4B.

FIG. 6 shows various signals for providing electrical energy to a loadfrom FIG. 1 in a normal state and in a short-circuit and/or overcurrentstate.

FIG. 7 shows a further example of a device for producing a dynamicreference signal.

FIG. 8 shows various signals in the device for producing a dynamicreference signal from FIG. 7.

DETAILED DESCRIPTION

The following description presents numerous details to enable a profoundunderstanding of the present invention. It is clear to the personskilled in the art, however, that the specific details are not necessaryto implement the present invention. Elsewhere, known devices and methodsare not portrayed in detail, in order that understanding the presentinvention is not made more difficult unnecessarily.

In the present description, a reference to “an embodiment”, “aconfiguration”, “an example” or “example” means that a specific feature,a structure or property that is described in association with thisembodiment is included in at least one embodiment of the presentinvention. In this regard, the phrases “in one embodiment”, “an example”or “example” at various points in this description do not necessarilyall refer to the same embodiment or the same example. Furthermore, thespecific features, structures or properties can be combined in arbitrarysuitable combinations and/or subcombinations in one or more embodimentsor examples. Particular features, structures or properties can beincluded in an integrated circuit, in an electronic circuit, in acircuit logic or in other suitable component parts which provide thefunctionality described. Furthermore, it is pointed out that thedrawings serve the purpose of elucidation for the person skilled in theart, and that the drawings are not necessarily drawn in a manner true toscale.

The above-described differences in the profile of the collector-emittervoltage between normal operation and short-circuit and/or overcurrentoperation can be utilized in protective circuits in order to detect ashort-circuit or overcurrent state. By way of example, if thecollector-emitter voltage rises above the threshold value (see FIG. 6,bottom right), a short-circuit or overcurrent state can be detected.However, the fall in the collector-emitter voltage need not bemonotonic. As likewise illustrated in FIG. 6, said fall can have voltagepeaks and voltage valleys—precisely just after the switch-on of thepower semiconductor switch. In the case of a static reference voltagethat can have the effect that the protective circuit erroneously assumesa short-circuit and/or overcurrent case and switches off the powersemiconductor switch. Likewise, in the case of power semiconductorswitches having a comparatively sluggish switchover behavior, the timeuntil a static state is reached may be comparatively long (e.g.significantly longer than 10 μs). A static reference voltage that is setnear a specific static forward voltage of the power semiconductor switchcan thus likewise have the effect that the protective circuiterroneously assumes a short-circuit and/or overcurrent case and switchesoff the power semiconductor switch. In this case, the protective circuitmay detect a dangerously increased voltage even though the voltage levelincreased at a specific point in time should merely be ascribed to theinertia of the power semiconductor switch.

FIG. 1 shows a device 100 (also designated as power converter) forproviding electrical energy to a load 110. However, the flow of energycan also point in the other direction. Element 110 is then a generator.In still other devices, element 110 in different operating states canoperate both as load and as generator. Hence reference is made merely toa device for providing energy, which encompasses all the cases justdiscussed (the energy can be provided at different outputs). The devicecomprises two power semiconductor switches 104, 106, which are connectedin series. In addition, device 100 can receive a DC input voltage 102(U_(IN)). The device is configured to transmit electrical energy by thecontrol of the power semiconductor switches 104, 106 from the input toan output to which the load 110 is connected (or in the oppositedirection). In this case, the device for providing electrical energy cancontrol voltage levels, current levels or a combination of bothquantities that are output to the load.

In the example in FIG. 1, the power semiconductor switches 104, 106 areIGBTs. Hence the devices and methods are explained on the basis of theexample of IGBTs. However, the devices for producing a dynamic referencesignal, the control circuits and the devices according to the inventionfor providing electrical energy are not restricted to the use withIGBTs. Rather, they can also be used in combination with other powersemiconductor switches. By way of example, metal oxide semiconductorfield effect transistors (MOSFETs), bipolar transistors, IEGTs(“injection enhancement gate transistors”) and GTOs (“gate turn-offthyristors”) can be used with the devices for producing a dynamicreference signal, the control circuits and the devices according to theinvention for providing electrical energy. Moreover, the devices fordetecting a profile of a voltage across a power semiconductor switch,the control circuits and the devices for providing electrical energy canbe used with power semiconductor switches which are based on galliumnitride (GaN) semiconductors or silicon carbide (SiC) semiconductors.

A maximum nominal collector-emitter, anode-cathode or drain-sourcevoltage of a power semiconductor switch in the switched-off state can bemore than 500 V, preferably more than 2 kV.

In addition, the devices for producing a dynamic reference signal, thecontrol circuits and the devices according to the invention forproviding electrical energy are not restricted to power semiconductorswitches. In this regard, it is also possible to use other semiconductorswitches with the devices for detecting a profile of a voltage across apower semiconductor switch, the control circuits and the devices forproviding electrical energy. The effects and advantages which arediscussed here also occur at least in part in systems with othersemiconductor switches.

Since IGBTs are thus discussed, the terminals of the power semiconductorswitch are designated as “collector”, “gate” and “emitter”. As alreadyexplained above, however, the devices and methods are not restricted toIGBTs. In order to avoid unnecessary lengths, the designation “emitter”herein also encompasses the terminal of corresponding powersemiconductor switches that is designated by “source” or “cathode”.Equally, the term “collector” herein also encompasses the terminaldesignated by “drain” or “anode”, and the term “gate” encompasses theterminal of corresponding power semiconductor switches that isdesignated by “base”. Hence the term “collector-emitter voltage” alsoencompasses a “drain-source voltage” and a “cathode-anode voltage”, andthe terms “collector voltage” and “emitter voltage” also encompass a“drain voltage” or “anode voltage” and respectively a “source voltage”or “cathode voltage”.

The power semiconductor switches 104, 106 are respectively controlled bya first and second control circuit 118, 120 (an exemplary controlcircuit is explained in connection with FIG. 2). Said control circuitsprovide a first and a second gate-emitter driver signal 130, 132(U_(GE1), U_(GE2)) in order to control the switching instants of thefirst and second IGBTs. Both control circuits 118, 120 can optionally becontrolled in turn by a system controller 114. The system controller 114can have an input for receiving system input signals 116. Two powersemiconductor switches 104, 106 in a half-bridge configuration areillustrated in the example in FIG. 1. The devices for producing adynamic reference signal, the control circuits and the devices forproviding electrical energy can also be used in other topologies,however. By way of example, an individual power semiconductor switch(e.g. an individual IGBT) can be connected up to a device for producinga dynamic reference signal or a control circuit. In other examples, in athree-phase system having six power semiconductor switches or twelvepower semiconductor switches, each of the power semiconductor switchescan have a device for producing a dynamic reference signal.

Besides outputting a gate-emitter driver signal, the control circuits118, 120 take up signals representing voltages present across the powersemiconductor switches 104, 106. The signals can be voltage signals orcurrent signals. In the example in FIG. 1, each control circuit 118, 120has a respective signal that is representative of the collector-emittervoltage and is designated as collector-emitter voltage signal 122, 124(U_(CE1), U_(CE2)).

In FIG. 1 and subsequently, circuits are described which use signalsrepresenting voltages present across the power semiconductor switches104, 106 (for example a collector-emitter voltage signal). As explainedfurther above, the indirect monitoring of the current through the powersemiconductor switch can be effected on the basis of the voltagespresent across a power semiconductor switch. However, a current throughthe power semiconductor switch (e.g. a collector current, a draincurrent or a cathode current) can also be estimated in some other way orbe directly measured. In one example, a device (also designated as powerconverter) for providing electrical energy to a load can have adetection circuit for directly measuring a current through a powersemiconductor switch (e.g. a device for measuring a collector current).

In FIG. 1, the control circuits 118, 120 are schematically depicted asseparate control circuits. However, both control circuits 118, 120 canalso be combined in a single circuit. In this case, a single controlcircuits controls two power semiconductor switches 104, 106.Furthermore, the second gate-emitter driver signal 132 (U_(GE2)) can bean inverted first gate-emitter driver signal 130 (U_(GE1)).

Both control circuits 118, 120 comprise a device for producing a dynamicreference signal. With the aid thereof, on the basis of the respectivecollector-emitter voltage signal 122, 124 it is possible to ascertain ashort-circuit and/or overcurrent state of the respective powersemiconductor switch. In reaction to ascertaining a short-circuit and/orovercurrent state, the respective power semiconductor switch 104, 106can be switched off.

FIG. 2 illustrates a control circuit 218 (for example one of the controlcircuits 118, 120 from FIG. 1). The control circuit 218 includes fourfunctional units: a device for producing a dynamic reference signal 242,a short-circuit and/or overcurrent state detection circuit 240, a drivercircuit 236 and an optional driver interface 234. The device forproducing a dynamic reference signal 242 is adapted to receive agate-emitter driver signal 230 from the driver circuit 236. However, adevice for producing a dynamic reference signal 242 can also receiveother signals representative of the gate-emitter driver signal 230. Itis merely necessary that information regarding the point in time of aswitchover process of the controlled power semiconductor switch from aswitched-off state into a switched-on state can be inferred from thesignal (and optionally information regarding the point in time of aswitchover process of the controlled power semiconductor switch from aswitched-on state into a switched-off state). In addition, the devicefor producing a dynamic reference signal 242 is optionally configured toreceive an external reference signal. In the example in FIG. 2, theexternal reference signal can be a reference signal 226 that isrepresentative of the emitter voltage of a power semiconductor switch.However, alternatively, as external reference signal it is also possibleto use other voltage levels that are offset by a constant offsetrelative to the emitter voltage of a power semiconductor switch.

In addition, the device for producing a dynamic reference signal 242 isconfigured to receive a constant-current signal 246 (I₁). In the examplein FIG. 2, the constant-current signal 246 is provided by the drivercircuit 236. The constant-current signal 246, too, can alternatively beprovided by other functional units of the control circuit 218 or else byan external source or be produced internally by the circuit forproducing the reference signal itself. Any suitable source of current isappropriate for producing the constant current 246.

On the basis of the received signals (that is to say theconstant-current signal 246 and the gate-emitter driver signal 230), thedevice for producing a dynamic reference signal 242 provides a dynamicreference signal 248. The latter is received by the short-circuit and/orovercurrent state detection circuit 240. The dynamic reference signal248 has a steady-state signal level after a predetermined time haselapsed after a switchover process of the power semiconductor switch. Ifthe power semiconductor switch switches from an OFF state into an ONstate, the dynamic reference signal 248 is increased above thesteady-state signal level at least in a part of a time period startingfrom the switchover process of the power semiconductor switch until thepredetermined time has elapsed. However, the steady-state signal levelof the dynamic reference signal need not be identical in all operatingsituations. In this regard, drifting or a slow variation of thesteady-state signal level in relation to a length of a switching cycleis possible. Moreover, the steady-state signal level can be settable.

The short-circuit and/or overcurrent state detection circuit 240, forits part, is configured to receive a signal 222 corresponding to thecollector-emitter voltage U_(CE) of a power semiconductor switch (forexample signals 122 or 124 from FIG. 1). On the basis of said signal 222corresponding to the collector-emitter voltage of the powersemiconductor switch, the short-circuit and/or overcurrent statedetection circuit 240 determines a signal representative of acollector-emitter voltage across the power semiconductor switch(U_(C′E)). In other examples, the short-circuit and/or overcurrent statedetection circuit 240 determines said signal from other signals orreceives a signal representative of a collector-emitter voltage acrossthe power semiconductor switch from a further circuit block. In furtherexamples, the short-circuit and/or overcurrent state detection circuit240 receives a signal representative of a current through the powersemiconductor switch (e.g. a collector current). As explained inconnection with FIG. 1, the signal representative of a current throughthe power semiconductor switch can be produced by a direct measurementof the current through the power semiconductor switch. The short-circuitand/or overcurrent state detection circuit 240 can then compare thesignal representative of a current through the power semiconductorswitch with the dynamic reference signal in order to detect a presenceof a short-circuit and/or overcurrent state in the power semiconductorswitch. Exemplary profiles of a signal representative of a currentthrough the power semiconductor switch are shown at the bottom in FIG. 6(on the basis of the example of the collector current I_(C′)), where itcan also be discerned that the normal and short-circuit and/orovercurrent states can be detected both directly from a characteristicchange in the profile of the current and indirectly from acharacteristic change in the profile of the voltage.

In response to the received or determined signal representative of acollector-emitter voltage across the power semiconductor switch (or areceived or determined signal representative of the collector currentthrough the power semiconductor switch) and the received dynamicreference signal 248, the short-circuit and/or overcurrent statedetection circuit 240 determines whether a short-circuit and/orovercurrent state is present.

In one example, the short-circuit and/or overcurrent state detectioncircuit 240 detects that a short-circuit and/or overcurrent state ispresent if the signal representative of the collector-emitter voltage(or the signal representative of the collector current through the powersemiconductor switch) is greater than the dynamic reference signal 248.As already described further above, the collector-emitter voltage (orthe collector current) should fall rapidly to a relatively low valueafter the power semiconductor switch has been switched on, and acollector-emitter voltage increased above said value (or an increasedcollector current) is an indirect (or direct) indicator of the presenceof a short-circuit and/or overcurrent state. The lower curves in FIG. 6illustrate this and the operation of the short-circuit and/orovercurrent state detection circuit 240. A normal switch-on process isillustrated on the left-hand side of FIG. 6. The voltage U_(C′E) (inthis case the signal representative of the collector-emitter voltage)falls rapidly after the switching process. The current I_(C′) (in thiscase the signal representative of the collector current) likewisequickly reaches a more or less steady-state level. The dynamic referencesignal always lies above the signal U_(C′E) 678 (or above a signalrepresentative of the collector current), such that no short-circuitand/or overcurrent state is ascertained. As already mentioned, thetemporary increase in the level of the dynamic reference signal afterthe switchover process can prevent an erroneous detection of ashort-circuit and/or overcurrent state.

By contrast, FIG. 6 schematically depicts at the bottom on theright-hand side a typical profile of the signal representative of thecollector-emitter voltage and the profile of the signal representativeof the collector current in a short-circuit and/or overcurrent case. Itcan be discerned here that the signal U_(C′E) 678 and the signal I_(C′)exceed the dynamic reference signal (e.g. after 8 to 12 μs after aswitchover process) and the short-circuit and/or overcurrent statedetection circuit 240 thus detects a short-circuit and/or overcurrentstate. As explained above, the dynamic reference signal is increased forat least one part of a predetermined time (for example for the entirepredetermined time), such that no fault is detected if during thepredetermined time voltage peaks and voltage valleys occur after theswitch-on of the power semiconductor switch, or the complete falling ofthe signal representative of the collector-emitter voltage to a lowvalue below the static reference signal takes longer, in principle, thanthe maximum permissible short-circuit duration defined for the powersemiconductor switch. The duration of the increase in the dynamicreference signal can be set in order to minimize the frequency of theoccurrence of incorrect fault detections. As can be seen in FIG. 6, asufficiently great rise in the signal representative of thecollector-emitter voltage or in the signal representative of thecollector current through the power semiconductor switch, despite thedynamic increase in the reference signal during the predetermined time,can trigger the detection of a short-circuit and/or overcurrent state.It may just be that the point in time of the detection is shifted backby the dynamic increase in the reference signal.

The overcurrent state detection circuit 240, if it has detected ashort-circuit and/or overcurrent state, can output a fault signal. Inthe example in FIG. 2, a fault signal 244 (U_(FT)) is output to thedriver circuit 236. The latter can switch off the power semiconductorswitch in reaction to the fault signal 244, in order to prevent damageto said power semiconductor switch. However, in alternativearrangements, the fault signal can also be processed in some other way.By way of example, it is possible just to supply the fault signal 250(U_(FT)) to a further control element (for example to the systemcontroller 214). Said further element can then switch off two or morepower semiconductor switches in a predetermined sequence.

In order to determine the fault signal, the short-circuit and/orovercurrent state detection circuit 240 can process further the receivedsignals. By way of example, the short-circuit and/or overcurrent statedetection circuit 240 can comprise a gating circuit in order to set apredetermined delay of the detection of a short-circuit and/orovercurrent state. Additionally or alternatively, the short-circuitand/or overcurrent state detection circuit 240 can contain a circuit forsetting a voltage level of the signal representative of thecollector-emitter voltage (or of a signal representative of thecollector current of the power semiconductor switch). Since thecollector-emitter voltage itself can be a few thousand volts or more,this circuit can comprise a network having high-value resistors.However, the network can also comprise further elements configured forhigh voltages (e.g. capacitive components or diodes). This produces fromthe collector-emitter voltage a signal that is proportional to thecollector-emitter voltage but has a (much) lower voltage level (forexample the signal U_(C′E) depicted schematically in FIG. 6).

As already described, the driver circuit 236 can receive a fault signal244 signaling a short-circuit and/or overcurrent state and can switchoff the power semiconductor switch in reaction to a detectedshort-circuit and/or overcurrent state. One or a plurality of componentsof the short-circuit and/or overcurrent state detection circuit 240 canoptionally be included in the driver circuit 236. By way of example, thedriver circuit 236 can receive the dynamic reference signal U_(REF) 248and the signal representative of the collector-emitter voltage (or asignal representative of the collector current through the powersemiconductor switch). In addition, the driver circuit 236 can alsosupply a gate-emitter driver signal for controlling the powersemiconductor switch.

In the example in FIG. 2, the driver circuit 236 is connected via agalvanic isolation 238 (e.g. a transformer) to the optional driverinterface 234, in order to receive control signals from the systemcontroller 214. The driver interface 234 can in turn be connected to thesystem controller 214, which receives system inputs 216, and iscontrolled thereby.

FIG. 3 illustrates an exemplary device for producing a dynamic referencesignal 342 (for example block 242 in FIG. 2) for a control circuit for apower semiconductor switch. The device for producing a dynamic referencesignal 342 contains two functional blocks, a reference signal generator352 and a passive charging circuit 350. The function of these blockswill be explained below. However, the exemplary circuit is notrestricted to cases in which the functions explained below are performedin separate units. Rather, a portion or all of the functions of a blockcan also be implemented jointly.

The device for producing a dynamic reference signal 342 receives acontrol signal of the power semiconductor switch 330 (U_(GE)) and aconstant-current signal 346 (I₁).

In FIG. 3 and in the subsequent figures, the devices for producing adynamic reference signal receive a constant-current signal. As alreadyexplained further above, other configurations are also possible. In oneexample, the devices for producing a dynamic reference signal receives aconstant-voltage signal.

The control signal of the power semiconductor switch 330 (U_(GE)) can beany signal representing the temporal sequence of the switchoverprocesses of the power semiconductor switch. By way of example, as shownin FIG. 1, the gate driver signal can be used. However, (primarily in acomplex IGBT driver circuit) a suitable signal can also be presentelsewhere in order to supply the device for producing a dynamicreference signal 342 with information about the switchover processes ofthe power semiconductor switch. The constant-current signal 346 (or aconstant-voltage signal) can also be produced in the device forproducing a dynamic reference signal 342 itself.

The reference signal generator 352 of the device for producing a dynamicreference signal 342 makes available a dynamic reference signal 348(U_(REF) or I_(REF)) having a steady-state signal level after apredetermined time has elapsed after a switchover process of the powersemiconductor switch. By way of example, the reference signal generator352 can comprise one or a plurality of components for producing thesteady-state signal level in response to the constant-current signal 346(or a constant-voltage signal). Examples of the reference signalgenerator 352 are shown in connection with FIG. 4A, FIG. 4B and FIG. 7.

The passive charging circuit 350 is configured to increase an around asignal level of the dynamic reference signal in reaction to a switchoverof a control signal of the power semiconductor switch from an OFF stateinto an ON state at least for a part of the predetermined time (forexample for the entire predetermined time) above the steady-state signallevel in order to produce a dynamic reference signal. By way of example,the passive charging circuit 350 receives the control signal U_(GE) 330.The control signal U_(GE) 330 is processed by the passive chargingcircuit 350 in order to determine the point in time of the switchoverprocess of a control signal of the power semiconductor switch from aswitched-off state into a switched-on state. In addition, the amplitudeand the temporal profile (that is to say also the length of the part ofor of the entire predetermined time in which the dynamic referencesignal 348 has an increased signal level) of the increase in the dynamicreference signal above the steady-state signal level can be determinedat least in part on the basis of the control signal U_(GE) 330. Thistemporary increase in the dynamic reference signal is brought about bythe passive charging circuit 350 without active components (such astransistors, for example). Exemplary passive charging circuits 350 arein turn shown in connection with FIG. 4A, FIG. 4B and FIG. 7.

In one example, the passive charging circuit 350 is configured tocontrol at least in part the temporal profile of the temporary increasein the dynamic reference signal as illustrated in FIG. 6. That comprisesan abrupt rise in reaction to a switchover of a control signal of thepower semiconductor switch from an OFF state into an ON state followedby a continuous fall back to the level of the steady-state voltagesignal. The time constant (1/e time constant) of the fall can be between1 and 50 μs (e.g. between 5 and 15 μs). In order to produce thistemporal behavior of the dynamic reference signal 348, the passivecharging circuit 350 can contain an RC element, wherein the timeconstant of a capacitance of the RC element determines at least in partthe speed of the fall to levels of the steady-state voltage signal. Inaddition, the rise in the level of the dynamic reference signal 348above the level of the steady-state reference signal can correlate witha jump in the amplitude of the control signal U_(GE) 330. Exemplarysignal levels are explained in connection with FIG. 6.

FIG. 4A shows a first example of a of a device for producing a dynamicreference signal 442. The device contains a reference signal generator452 and a passive charging circuit 450. In addition, the device forproducing a dynamic reference signal 442, as also in the example in FIG.3, receives a control signal of the power semiconductor switch 430(U_(GE)), a constant-current signal 446 (I₁). With regard to theselection of these signals and alternative signals, the statements madeat the corresponding point in the description of FIG. 3 are applicable.

Reference signal generator 452 contains a circuit for producing a staticreference signal. The circuit for producing a static reference signalincludes a reference resistor 464 (R_(REF)). The constant-current signal346 is coupled to a first terminal of the reference resistor 464, flowsthrough the reference resistor 464 and thus produces a constant voltagedrop (I₁*R_(REF)). Even though the reference resistor 464 is illustratedas an individual component in FIG. 4A, a combination of a plurality ofresistors can also be used.

In other examples, it is possible to use a constant-voltage source inthe devices for producing a dynamic reference signal (instead ofproducing the latter by the fall in the constant current I₁ across thereference resistor R_(REF) 464). In one example, a constant voltagesource can be connected in series with the reference resistor R_(REF)464 (as it were instead of V₁). For example, the constant voltage sourcecan be connected between the reference resistor R_(REF) and V_(E). Inother examples, an internal reference level already present in thedevices for producing a dynamic reference signal here can form thestatic voltage reference level (e.g. the internal reference level 447(V₁)). For example, the internal reference level can be produced by apassive semiconductor circuit (e.g. with a zener diode). In anotherexample, the reference resistor R_(REF) 464 can be omitted. The devicesfor producing a dynamic reference signal then produces a referencecurrent I_(REF) (instead of a reference voltage U_(REF)).

The device for producing a dynamic reference signal 442 produces a firstinternal reference level 447 (V₁). For example, the first internalreference level 447 (V₁) can be produced on the basis of the referencelevel 426 or correspond thereto. In one example, the first internalreference level 447 is substantially equal to the emitter voltage of thepower semiconductor switch. That is advantageous since typically areference signal representative of the emitter voltage is likewise usedin short-circuit and/or overcurrent state detection circuits (see block240 in FIG. 2). Consequently, in this case both a collector-emittervoltage signal and a dynamic reference signal, which are compared withone another, are referenced to the same signal. That can facilitate themutual stabilization of the signals. Nevertheless, any level lying belowthe static voltage reference level is usable for the first internalreference level 447 (V₁).

The node A of the reference signal generator 452 which is connected toan output for outputting the dynamic reference signal 448 is thus raisedto a steady-state signal level by the circuit for producing thesteady-state signal level. In FIG. 4A and the subsequent figures, thedynamic reference signal 448 (U_(REF)) is a dynamic voltage signal.However, as already explained a number of times, a dynamic currentsignal can also be used. This steady-state signal level is V₁+R_(REF)*I₁in the exemplary device in accordance with FIG. 4A. If the voltage levelof the node A is not influenced by the passive charging circuit 450, thethe level of the dynamic reference signal 448 corresponds to thesteady-state signal level (that is to say V₁+R_(REF)*I₁). This behavioris illustrated in FIG. 5A. The bottommost curve represents an exemplaryprofile of the voltage at the node A (and thus of the dynamic referencesignal 548). It can be seen that the voltage level at the node A is inthe steady state apart from specific times following the switchoverprocesses of the semiconductor switch.

The passive charging circuit 450 is connected to the reference signalgenerator 452 via resistor 466 (R₁) (the latter is depicted as beingassociated with the reference signal generator 452 in FIG. 4A, but thatis an arbitrary classification). The passive charging circuit 450contains an RC element. The latter comprises a capacitance 456 (C_(T))and a resistor 460 (R_(T)). The capacitance 456 is coupled between anode B and an input that receives the control signal of the powersemiconductor switch 430. The resistor 460 is coupled between the node Band the node A of the reference signal generator 452. The resistors 460and 466 can also be an individual resistor.

Moreover, the passive charging circuit 450 contains an optional firstclamping circuit 454, which limits a voltage level at the node B to asecond internal reference level 462 (V_(L)). In the example in FIG. 4A,the first clamping circuit 454 contains a diode 454. The second internalreference level 462 can be any level that is less than or equal to thestatic voltage reference level. If the external reference level 426fulfills this condition, it can be used as second internal referencelevel 462. The second internal reference level 462 can thus in turn berepresentative of the emitter voltage of the power semiconductor switch.The passive charging circuit 450 contains no active components.

The function of the passive charging circuit will be explained againwith reference to FIG. 5A. As already discussed, the device forproducing a dynamic reference signal 442 outputs directly before aswitchover process a dynamic reference voltage substantiallycorresponding to the internal reference level 447 plus the voltagegenerated across resistor R_(REF) 464 by constant current 446(I₁*R_(REF)+V₁ in the device in FIG. 4A). In other words, shortly beforea switchover process the dynamic reference voltage 448 is equal to thesteady-state voltage level. The topmost curve in FIG. 5A shows anexemplary profile of a control signal 530 (U_(GE)) of the powersemiconductor switch. The voltage level of the control signal 530 risesfrom a level in the switched-off state V_(OFF) to a level in theswitched-on state V_(ON). In principle, these levels can be chosenarbitrarily. In one example, the levels V_(OFF) and V_(ON) correspond tothe gate-emitter voltage of the power semiconductor switch, whichcontrols the power semiconductor switch, in the switched-off andswitched-on state, respectively. For example, a level in theswitched-off state V_(OFF) for IGBTs can lie between −20V and −5V(preferably between −15V and −7V) and a level in the switched-on stateV_(ON) for IGBTs can lie between 10V and 20V (preferably between 13V and15V). For power MOSFETs, the levels V_(OFF) and V_(ON) cancorrespondingly correspond to the voltage of the gate-source voltage ofthe power MOSFET in the switched-off and switched-on state,respectively. For example, a level in the switched-off state V_(OFF) forpower MOSFETs can lie between −5V and 0V and a level in the switched-onstate V_(ON) for MOSFETs can lie between 10V and 25V. For SiC-basedcomponents, a level in the switched-off state V_(OFF) can likewise liebetween −5V and 0V and a level in the switched-on state V_(ON) can liebetween 10V and 20V.

As shown in FIG. 5A, the voltage level at the node B and the node A ofthe device for producing a dynamic reference signal 442 rises abruptlyat the point in time of the switchover from a switched-off state into aswitched-on state of the power semiconductor switch. This risecorresponds to the voltage swing of the control signal 530 (controlsignal 430 in FIG. 4A) upon the switchover from the switched-off stateinto the switched-on state. That can be comprehended with reference toFIG. 4A: since the voltage swing of the control signal 430 increases thevoltage level at an output of the capacitance 456, the voltage level atthe node B also increases by this absolute value (the diode 454 isnonconducting since the level at node B lies above the level of thesecond reference voltage). The amplitude of the voltage jump thuscorresponds to the difference between the level in the switched-on stateV_(ON) and the level in the switched-on state V_(OFF) of the controlsignal 530 (430 in FIG. 4A). Consequently, at the node B at the point intime after the switchover process this results in a voltage levelV_(Bpeak) of

V _(Bpeak) V _(ON) +I ₁ R _(REF) +V ₁ −V _(OFF′)  (Equation 1)

This has the consequence that an additional current is driven throughthe resistors 460 (R_(T)), 466 (R₁) and 464 (R_(REF)). This currentincreases the voltage level at the node A by a specific absolute value(namely the current intensity of the additional current multiplied bythe value of the resistor 464). Consequently, directly after theswitch-on of the power semiconductor switch at the node A this resultsin a voltage of

$\begin{matrix}{V_{Apeak} = \frac{{\left( {{I_{1}R_{REF}} + V_{1}} \right)\left( {R_{1} + R_{T}} \right)} + {V_{Bpeak}R_{REF}}}{R_{REF} + R_{1} + R_{T}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

Since the output for tapping off the dynamic reference signal 448 isconnected directly to the node A, the voltage level shown in FIG. 5Acorresponds to the dynamic reference signal 448.

In addition, the additional current discharges the capacitance 456 ofthe passive charging circuit 450. That has the consequence that avoltage across the capacitance 454 decreases and, consequently, thevoltage level at the node B also falls. It follows from that in turnthat the additional current decreases and the voltage at the node A alsofalls (and thus so does the level of the dynamic reference signal 448).After a predetermined time, the capacitance has discharged to an extentsuch that no more additional current is driven through the resistor 464.Consequently, the voltage at the node A is again at its steady-statelevel (of I₁*R_(REF)+V₁). The temporary increase in the dynamicreference signal that is produced by the passive charging circuit hasdecayed.

The time constant of the discharge process of the capacitance 456 isdetermined by the values of the resistors 460 (R_(T)), 466 (R₁) and 464(R_(REF)) and the value of the capacitance 456 (C_(T)). It and theamplitude of the voltage swing in the dynamic reference signal 448 canbe chosen such that erroneous detections of an a short-circuit and/orovercurrent state after a switchover process from a switched-off stateinto a switched-on state of the power semiconductor switch are preventedor at least reduced and a protection function is nevertheless achievedby means of a reliable switch-off of the power semiconductor switch.That can in turn be illustrated with reference to FIG. 6. The temporaryincrease in the dynamic reference signal 648 ensures that the signalrepresentative of the collector-emitter voltage lies below the dynamicreference signal in the normal case (left-hand side in FIG. 6). Theshort-circuit and/or overcurrent state detection circuit 240 in FIG. 2would thus not detect a short-circuit and/or overcurrent state.

FIG. 5A additionally shows the behavior of the device for producing adynamic reference signal 442 after a switchover process from aswitched-on state into a switched-off state of the power semiconductorswitch. Here the opposite process occurs relative to the case of theswitchover process from the switched-off state into a switched-on stateas was described further above. Once again a, in this case negative,voltage swing of the control signal 530 is present at the first input ofthe capacitance 454 and reduces the voltage at the node B. This in turnresults in an additional current that reduces the voltage at the node A.In contrast to the switch-on process, however, now the first clampingcircuit 454 is active and limits the voltage drop at the node B to thesecond internal reference level V_(L). As already mentioned, the firstclamping circuit 454 is optional; the device for producing a dynamicreference signal 442 can also be used without it. The dynamic referencesignal 448 would then be reduced by the full voltage swing of thecontrol signal 430. That may be disadvantageous, however, since undercertain circumstances the voltage at the node A (the dynamic referencesignal 448) and the voltage at the node B do not have enough time toreturn to the steady-state signal level.

The voltage swing at the node A, which is downwardly limited by thefirst clamping circuit, thus results as:

$\begin{matrix}{V_{Aclamp} = \frac{\left( {{I_{1}R_{REF}} + V_{1}} \right)\left( {R_{1} + R_{T}} \right)V_{L}R_{REF}}{R_{REF} + R_{1} + R_{T}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

Analogously to the situation described above, the additional currentthen has the effect that the capacitance 456 charges. At the end of thischarging process, the voltage 458 across the capacitance 456 againcorresponds to the voltage before the beginning of the switchoverprocess, the additional current stops flowing and the dynamic referencevoltage 548 again has its steady-state signal level (I₁*R_(REF)+V₁).

FIG. 4B shows a second example of a device for producing a dynamicreference signal. The construction corresponds to that of the deviceshown in FIG. 4A in which a second clamping circuit 468, 470 isadditionally provided. Said second clamping circuit is configured tolimit the dynamic reference voltage 448 to a predetermined maximumvalue. In FIG. 4B the maximum value of the dynamic reference voltage 448is defined by a third internal reference level V_(H). Since the secondclamping circuit 468, 470 is coupled between the resistors 460 and 466,the maximum value of the voltage V_(apeak) at the node A is clamped to

$\begin{matrix}{V_{Apeak} = \frac{{\left( {{I_{1}R_{REF}} + V_{1}} \right)R_{1}} + {V_{H}R_{REF}}}{R_{REF}R_{1}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$

In order that the illustration is not made more complicatedunnecessarily, for the equations discussed here a voltage drop acrossthe diodes of zero was assumed if the diodes are forward-biased. Thesecond clamping circuit 468, 470 can contain a second diode 468. Thethird internal reference level V_(H) is chosen to prevent damage tocomponents to which the device 442 for producing a dynamic referencesignal is connected. By way of example, as shown in FIG. 2, the constantcurrent I₁ can be supplied by the driver circuit 236. Since the dynamicreference signal 448 is present at the reference output of the drivercircuit 236, the latter can incur damage as a result of the increasedvoltage level of the dynamic reference signal 448. For example, themaximum permissible voltage at the reference output of the drivercircuit 236 may correspond to V_(ON). In other environments, the maximumpermissible voltage may also be higher, for which reason a limitation bymeans of the second clamping circuit 468 is not absolutely necessary.

The profile of the temporarily increased dynamic reference signal 448 inthe device in accordance with FIG. 4B is depicted schematically in FIG.5B. It can be seen here that the profile of the voltage level at thenode B in the devices in accordance with FIG. 4A and FIG. 4B isidentical. The profile of the temporary reduction of the dynamicreference signal 548 below its steady-state level also corresponds tothat depicted schematically in FIG. 5A. A difference can be seen in theswitchover process from a switched-off state into a switched-on state ofthe power semiconductor switch. Here the dynamic reference signal isfirstly clamped to the value given in equation 4 until the diode 468 ofthe second clamping circuit stops conducting. Afterward, the dynamicreference voltage 548 falls again to its steady-state signal level.

In FIG. 7 there is a third example of a device for producing a dynamicreference signal 742. The device corresponds to the device 442 shown inFIG. 4B, with the exception that a third diode 772 (D₃) is arrangedbetween the second clamping circuit 768, 570 and the resistor 760(R_(T)). The third diode 772 (D₃) influences the behavior of the devicefor producing a dynamic reference signal 742 after a switchover processof the power semiconductor switch from a switched-on state into aswitched-off state. As can be seen in FIG. 8, the profiles of thevoltage levels at the nodes A (corresponding to the dynamic referencesignal 848) and the voltage level at the node B 855 after the switch-onprocess correspond to those shown in FIG. 5B. After a switchover processfrom a switched-on state to a switched-off state of the powersemiconductor switch, the behavior of the devices for producing adynamic reference signal in accordance with FIG. 4B and FIG. 8 differs,however.

If the negative voltage swing of the control signal 830 reduces thevoltage level at node B (limited in turn by the first clamping circuit754 to the voltage level V_(L)), the diode 772 (D₃) is turned off. Thereference signal generator 752 is thus decoupled from the passivecharging circuit 752. That in turn has the consequence that a fall inthe voltage level at the node A below the steady-state signal level (andthus also in the dynamic reference signal 748) can be prevented orconsiderably reduced. The voltage level at the node A returns veryrapidly to its static level (V₁+I₁*R_(REF)) since this process is nolonger coupled to the discharge behavior of the capacitance 756 of theRC element of the passive charging circuit 750. The voltage U_(CT)across the capacitance 756 (C_(T)) after the change from the OFF stateinto the ON state is statically U_(CT)=(I₁*R_(REF)+V₁)−V_(ON). Thecapacitance is discharged to this value via R_(T), R₁ and R_(REF), whichleads to the dynamic increase in the dynamic reference voltage U_(REF).Upon the change from the on into the off state, the capacitance 756(C_(T)) is recharged to the value U_(CT)=V_(L)−V_(OFF) via the firstclamping circuit 754 without significant delay (no resistor is involvedin the charging; the diode D1 is disregarded here). Since the secondinternal reference level 762 (V_(L)) is lower than I₁*R_(REF)+V₁, thediode 772 (D3) is turned off. Consequently, the the capacitance 756(C_(T)) is not additionally recharged via this path. This behavior isillustrated in curve 848 in FIG. 8, which shows the temporal profile ofthe dynamic reference signal in the device in accordance with FIG. 8.

A suppression or reduction of the negative excursion (below thesteady-state voltage level) is advantageous since the negative excursionof the dynamic reference signal 748 for example in the case of short offperiods 876 (t_(OFF)) can have the consequence that the dynamicreference signal 848 has not yet returned to its static value again whena subsequent switch-on process takes place. That can have theconsequence that the dynamic reference signal U_(REF) is “compressed”upon switch-on (see FIG. 6, bottom left) in the dynamic (i.e. the peakvalue of the dynamic reference signal U_(REF) is lower than in the caseof longer off periods 876). That in turn, under certain circumstances,can have the effect that the signal U_(C′E) representative of thecollector-emitter voltage can cross the profile of the dynamic referencesignal U_(REF) even upon normal switch-on. The overcurrent statedetection circuit 240 then reports an overcurrent state or short-circuitstate to the driver circuit 236, even though nothing of the sort hasoccurred.

The above description of the illustrated examples of the presentinvention is not meant to be exhaustive or restricted to the examples.While specific embodiments and examples of the invention are describedherein for illustration purposes, various modifications are possiblewithout departing from the present invention. The specific examples forvoltage, current, frequency, power, range values, times, etc. are merelyillustrative, such that the present invention can also be implementedwith other values for these variables.

These modifications can be made to examples of the invention in light ofthe detailed description above. The terms used in the following claimsshould not be interpreted such that the invention is restricted to thespecific embodiments disclosed in the description and the claims. Thepresent description and the figures should be regarded as illustrativeand not as restrictive.

1. A device for producing a dynamic reference signal for a controlcircuit for a power semiconductor switch, wherein the device comprises:a reference signal generator for providing a dynamic reference signalhaving a steady-state signal level after a predetermined time haselapsed after a switchover process of the power semiconductor switch; apassive charging circuit, which is configured to increase a signal levelof the dynamic reference signal in reaction to a switchover of a controlsignal of the power semiconductor switch from an OFF state into an ONstate for at least one part of the predetermined time above thesteady-state signal level, in order to produce the dynamic referencesignal; and an output for tapping off the dynamic reference signal. 2.The device as claimed in claim 1, wherein the dynamic reference signalis a dynamic reference voltage.
 3. The device as claimed in claim 1wherein the passive charging circuit comprises an RC element.
 4. Thedevice as claimed claim 1, wherein the passive charging circuit isconfigured to increase the signal level of the dynamic reference signalin reaction to the switchover of the control signal in the manner of anabrupt jump.
 5. The device as claimed in claim 3, wherein the passivecharging circuit is further configured such that the capacitance of theRC element charges following a switchover of a control signal of thepower semiconductor switch from an ON state into an OFF state.
 6. Thedevice as claimed in claim 3, wherein the passive charging circuit isconfigured such that the capacitance of the RC element dischargesfollowing the switchover of the control signal and thus produces thetemporarily increased level of the dynamic reference signal and returnsit to the steady-state level again as a result of the discharge.
 7. Thedevice as claimed in claim 1, wherein the reference signal generatorcontains a circuit for producing the steady-state signal level.
 8. Thedevice as claimed in claim 1, further comprising: a first clampingcircuit, which is configured to limit the dynamic reference signal to apredetermined minimum level.
 9. The device as claimed in claim 1,further comprising: a second clamping circuit, which is configured torestrict the dynamic reference signal to a predetermined maximum level.10. The device as claimed in claim 1, further comprising: a passiveswitch, which is coupled between the reference signal generator and thepassive charging circuit, wherein the passive switch is configured toisolate the reference signal generator and the passive charging circuitin reaction to a switchover of the control signal of the powersemiconductor switch from an ON state into an OFF state.
 11. The deviceas claimed in claim 1, wherein the dynamic reference signal is a currentsignal.
 12. The device as claimed in claim 1, wherein the circuit forproducing a dynamic reference signal an input for coupling to anexternal static reference level, and wherein the circuit for producing adynamic reference signal is configured to use the static reference leveldirectly or in converted form as the steady-state signal level.
 13. Thedevice as claimed in claim 1, wherein an internal reference level of thedevice forms the steady-state signal level.
 14. The device as claimed inclaim 3, wherein the capacitance of the RC element discharges with atime constant selected in order to prevent the short-circuit and/orovercurrent state detection circuit from detecting a short-circuitand/or overcurrent state in normal operation.
 15. The device as claimedin claim 3, wherein the capacitance of the RC element discharges with atime constant selected in order to ensure that the short-circuit and/orovercurrent state detection circuit detects a short-circuit and/orovercurrent state in the short-circuit and/or overcurrent case.